1. Field of the Invention
The present invention relates to an image distortion compensating apparatus for compensating for an image distortion of a CRT (cathode ray tube) used in a television receiver or a display monitor, and more particularly to an image distortion compensating apparatus with a simple construction for compensating for a center pincushion distortion on left and right sides of a screen.
2. Description of the Related Art
When an image is formed on the front screen of a CRT of a television receiver by means of deflection of an electron beam, the image includes a pincushion distortion.
The pincushion distortion refers to a horizontal line distortion in a vertical direction or a vertical line distortion. Various methods and apparatuses for compensating for such distortions have been proposed.
However, even after compensating for such distortions, there can still be a residual center pincushion distortion (i.e., an "inner vertical-line pincushion distortion") on the front screen.
FIG. 1 is a front view of a front screen, which schematically illustrates a center pincushion distortion and the principle of compensating for such a distortion. In FIG. 1, O denotes the vertical center line of the screen.
In the CRT, if distortion is compensated for such that good linearity is obtained at the left and right ends of the screen, there can be a large residual nonlinear distortion near the center line O as represented by solid lines in FIG. 1.
A pincushion distortion compensating circuit for compensating for such a center pincushion distortion has been proposed (disclosed for example in Japanese Examined Patent Publication No. 5-3189).
This distortion compensating circuit is constructed as follows. That is, in a horizontal deflection circuit, which includes a horizontal deflection coil and a sigmoid distortion compensation capacitor and which is driven by a power supply modulated by a vertical synchronization parabolic-wave, a secondary winding of a voltage-current conversion transformer is connected between the horizontal deflection coil and the sigmoid distortion compensation capacitor and a voltage having a maximum value at the center of the vertical synchronization period is applied to the primary winding of the above-described transformer.
In recent years, as the front screen of CRTs becomes increasingly flat, the deflection angle tends to become greater.
Because upper and lower areas of the screen of the CRT are more distant from the deflection center than a central area of the screen, the upper and lower areas need a lesser amount of compensation for the sigmoid distortion than the central area.
Therefore, if the sigmoid distortion is compensated for using a conventional sigmoid distortion compensating circuit, the sigmoid distortion is overcompensated in the upper and lower areas compared to the central area.
This makes it difficult to completely compensate for the pincushion distortion represented by the solid lines in FIG. 1 and it becomes necessary to compensate for the sigmoid distortion to a greater degree. The difference in distance with respect to the deflection center between the upper/lower areas and the central area of the screen becomes greater and thus a greater distortion occurs.
To solve the above problem, an apparatus for compensating for the inner vertical-line pincushion distortion has been proposed (disclosed for example in Japanese Unexamined Patent Publication No. 9-149283).
In this image distortion compensating apparatus, the amount of sigmoid distortion compensation in the horizontal deflection is varied depending on the amount of vertical deflection by controlling the inductance of a saturable reactor, through which the horizontal deflection current is passed, depending on the vertical deflection current.
This image distortion compensating apparatus uses a saturable reactor including four reactor coils connected to each other in a series fashion and also connected in series to the horizontal deflection coil so that the horizontal deflection coil is passed through the four reactor coils, a magnet (i.e., a permanent magnet) around which the four reactor coils are wound such that magnetic biases are applied in opposite directions to two respective ends of the set of four reactor coils, and another coil which is connected in series to the vertical deflection coil so that the vertical deflection current is passed through this coil thereby controlling the inductance of the four reactor coils.
In. CRTs, as described above, it is required to prevent image quality degradation caused by center pincushion distortion (inner vertical-line pincushion distortion) which can occur on the left and right sides of the screen, and various techniques of compensating for such a distortion are proposed.
However, in the distortion compensating apparatus disclosed in Japanese Unexamined Patent Publication No. 9-149283, the saturable reactor needs four reactor coils (i.e., horizontal compensation coils).
Furthermore, the magnet for producing bias magnetic fields in opposite directions can cause a leakage magnetic field, and thus landing has to be taken into account.
Thus, it is an object of the present invention to provide a small-sized image distortion compensating apparatus with a simplified construction including a lesser number of horizontal compensation coils and requiring no particular consideration on the landing effects, capable of being produced at less cost.